The multiplexing of several telephone channels onto a single pulse-code modulated carrier is commonly carried out by a system standardized throughout the telephone industry and known as a T1-type system. D3 is the most commonly used T1-type system. In order to enable the receiving apparatus in such a system to properly synchronize itself with the incoming pulse train, D3 systems use a so-called D3 signal format, in which each one hundred ninety-two bit information segment of the pulse train is preceded by a framing pulse whose logic level is determined by an arbitrary standardized scheme.
Specifically, in the D3 format, the framing pulses alternate between main frame pulses whose sequence is 101010101010 . . . , and signalling frame pulses whose sequence is 000111000111 . . . , the initial "0" of the signalling frame pattern always following a "0" of the main frame pattern. It will be noted that the main frame pattern repeats every four frames, whereas the signalling frame pattern repeats every twelve frames.
On occasion, it is desirable to transmit over the T1-type carrier certain low-speed information such as equipment status or alarm information, without using one of the multiplexed channels for that purpose. U.S. Pat. No. 3,909,540 to Maryscuk et al shows one way of accomplishing this. Noting that the triple bits of the signalling frame pattern are redundant, and that 0x0x1x0x0x1x (where x can be either "1" or "0") is an equally distinctive pattern, Maryscuk derives the signalling frame synchronization from the 001 pattern of those signalling frame bits which immediately follow a "0" bit of the main frame pattern. This leaves every other signalling frame bit (i.e. the signalling frame bits immediately following the "1" bits of the main frame pattern) free for the transmission of low-speed information. The positions of these bits in the pattern are referred to herein as the "x" positions.
Inasmuch as the low-speed information must also be synchronized for proper decoding, the above-described prior art scheme provides so-called superframe framing bits in every third "x" position. The superframe framing pattern is chosen as 10001000 . . . ; consequently, with each superframe being twelve frames long, the low-speed frame pattern repeats every forty-eight frames and is capable of transmitting eight channels of low-speed data.
In the event of a loss of synchronization, the reframing time (i.e. the time required to re-synchronize all receiver frunctions with the incoming pulse train) is a multiple, determined by the specific reframing method being used, of the longest pattern repetition interval, i.e. forty-eight frames in the above-described prior art system.
It will be noted that in the prior art system, separate synchronization detectors must be provided for the signalling frame pattern (i.e. the 001001 . . . pattern) and the super-frame framing pattern (i.e. the 10001000 . . . pattern).
With the advent of more sophisticated equipment and the consequent need to transmit more and higher-frequency data, it is desirable or even critical to be able to piggy-back more low-speed data channels with a shorter reframing time and simpler equipment onto the D3 pattern.